Manufacturers of video display systems, such as televisions and VCRs are constantly searching for ways to improve the clarity of the displayed images. Presently, at least two problems stand in the way of designers of high definition video display systems. The first of these problems is corruption of pixels by impulse noise, impulse noise in general being caused by outside influences on the video display system. The second problem is the loss of image definition inherent with the use of interlaced video displays, presently the standard television display configuration in the United States.
In broadcast television applications, impulse noise is primarily the result of either the receipt of transmitted signals having a poor signal to noise ratio or spurious electromagnetic pulses, often generated by neighboring home appliances. When the television or video system is being used to play back images stored on a video cassette or disk, impulse noise is commonly the result of the media storing the images being worn or defective. In each case, the impulse noise degrades the displayed image by injecting the familiar small white dots or streaks in the image.
These problems are further complicated because video images are for the most part dynamic, depicting constantly moving or changing scenes. The display screen must therefore be constantly updated which in turn requires any filtering approach to operate dynamically and with sufficient speed to handle changes in the displayed images.
Under the current interlaced television format, each frame displayed consists of two "interlaced" fields formed by alternating rows of pixels, normally designated as the odd and even fields. The fields are alternately scanned at a rate of 60 Hz such that an entire image or frame is updated effectively at a frame rate of 30 Hz. With each scan the video components of intensity and chroma of each pixel in the field being scanned are updated to display the characteristics of the desired image. Through this updating, motion and other changes in the images are accounted for. The interlaced field format has a substantial disadvantage however. Discontinuities arise when one field is updated to display a new image while the other field remains set to the previous image. In essence, one field is always lagging behind the other due to the alternating scan.
One means of eliminating the problem of interlaced rows in a video display screen is the double line approach in which pairs of adjacent odd and even rows are simultaneously updated to display the same line of the image. While providing video component data for those pixels in the field not being scanned, this approach substantially lowers the resolution of the displayed image since each row in the scanned field is simply being doubled. Further, this approach alone does not simultaneously account for the problems relating to impulse noise.
A second approach, which may applied both to the problem of reduced definition from interlaced displays and the problem of pixels corrupted by impulse noise is median filtering. In median filtering, a median value is taken for the intensity and/or chroma levels (the video component values) for a given group of pixels in a selected neighborhood of a pixel being characterized. The video component value or values for the pixel being characterized are then set at the median value. The processing then moves on to another pixel where the same steps are repeated. This is performed for each pixel in the display screen. In this manner video component levels are made available for those pixels not in the field being updated as part of the current scan. Further, corrupted pixels in the field being currently scanned, which normally would exceed the median video values for the neighborhood, are eliminated by simple replacement with median. This approach has the serious disadvantage of blurring the displayed images since groups of pixels are always being set to the median. Most importantly, median filtering is computationally intensive thereby demanding maximum performance out of the video system. This is especially significant since preferably the filtering would be performed in real time.
The median filtering approach can be improved by dynamic "thresholding" such that only those pixel component levels exceeding the median value by more than a specified amount are actually replaced with the median. For a pixel in the field not being updated on the current scan, a comparison of the value of the selected video component level for that pixel, taken from the previous scan, can be made against the median value for that component as calculated for the neighborhood, and if the value for the previous level exceeds the neighborhood median, the pixel is set to the median level, otherwise the previous value for the previous level for that pixel is carried forward. For a pixel in the current scan, the value of current selected video component level is compared with the neighborhood median value and if that value exceeds the neighborhood median, such as in the case of a pixel corrupted by impulse noise, the pixel component level can be replaced with the component level associated with the median value, otherwise the level generated for the current scan is used.
Thus the need had arisen for improved devices, systems and methods for providing high definition video images. Such means should address video display definition problems arising both from pixels corrupted by impulse noise and from interlacing of pixels in the standard display format. Further, such means not be so computationally intensive as to make real time operation difficult.